Egyptian authorities have rounded up at least 40 rights workers, activists and lawyers since late October and detained them in "undisclosed locations", Human Rights Watch said Sunday.
"Many of those arrested were people who provided humanitarian and legal support to families of political detainees," HRW said, calling on the government to reveal their whereabouts.
The rights group spoke with a lawyer and three activists who had been "in direct touch" with the families of those detained, it said.
"The Egyptian security agencies' repression now extends to disappearing those brave men and women who have been trying to protect the disappeared," said Michael Page, deputy Middle East and North Africa director at Human Rights Watch.
One source told HRW as many as 80 people had been detained in the wave of arrests, but the rights group said it could only verify 40 names.
It called on Egyptian authorities to "immediately reveal all the detainees' whereabouts, release all of those arrested solely for exercising their rights, and bring any others swiftly before a judge to review their detention".
Sources told HRW that some of the detainees were close to the Egyptian Coordination for Rights and Freedoms, an independent human rights group it said has come under fire from pro-government media in recent months.
Among those arrested is Ms Hoda Abdelmoneim, a 60-year-old lawyer and former spokesperson for the Egypt's Women Revolutionary Coalition -- a group close to the Muslim Brotherhood.
Authorities also arrested Ms Aisha Khairat al-Shater, the daughter of a former Muslim Brotherhood leader currently in detention, along with her husband, lawyer Mohammed Abu Horayra.
The Brotherhood was outlawed and branded a "terrorist" organisation in December 2013, months after the military ousted Islamist president Mohamed Morsy following mass protests against his rule.
Former armed forces chief Abdel Fattah al-Sisi won the presidency in 2014 and has since launched a crackdown against the Brotherhood that has also hit Egypt's secular opposition.
Egypt's courts have sentenced hundreds of Brotherhood leaders and supporters to death or lengthy jail terms after other speedy mass trials.
Earlier this month, Amnesty International denounced the arrest of at least 19 lawyers and human rights activists, including Shater and Abdelmoneim.